1. Field of the Invention
The present invention is directed to the field of piezoelectric sensors and their use for detecting organic compounds. In particular, the present invention is directed to piezoelectric plate sensors capable of detecting organic compounds in a solution at very low concentrations.
2. Description of the Related Technology
There are many technologies that are capable of detecting biomolecules in a sample, such as quartz crystal microbalance-based technology, silicon microcantilever-based technology, electrochemical enzymatic immunoassays, fluorescence-based technology, laser-based or fiber-optics-based technology, amplification-based technology such as the polymerase chain reaction, and technology that tags metal particles to determine the presence of biomolecules. These technologies, however, fail to provide one or more of rapid, efficient or highly sensitive detection of biomolecules. In addition, many of them are also incapable of simultaneously detecting multiple biomolecules or being used in high throughput applications.
For example, quartz crystal microbalance-based technology, which utilizes thickness-mode resonance sensing, is one of the most commonly used biosensing technologies. Detection sensitivity of this technology is determined by its resonance frequency and the thickness of a quartz membrane. A resonance frequency of about 5 MHz, corresponding to a quartz membrane thickness of 330 μm, enables a minimum detectable mass density of about 10−9 g/cm2. Sensitivity is therefore generally limited to a range of about 10−8 g/Hz, which is not sufficiently sensitive for many biomedical applications.
To increase detection sensitivity, some sensors utilize silicon-based microcantilevers, which are said to offer a sensitivity of approximately 10−12 g/Hz, about four orders of magnitude higher than the quartz crystal microbalance-based technology. Silicon microcantilevers are also commercially available and may be easily integrated with existing silicon fabrication industrial processes. Silicon microcantilevers, however, generally rely on complex external optical devices for deflection detection, and an external driving mechanism for actuation and laser alignment, which make the silicon microcantilevers complex and expensive to use. Moreover, silicon microcantilevers are inferior for in-solution detection, in comparison with piezoelectric sensors.
Piezoelectric cantilevers, in comparison, are much simpler and easier to operate than the silicon-based microcantilever sensors. Piezoelectric cantilevers are typically constructed from lead zirconate titanate (PZT) and use electrical means for detection of biomolecules. Piezoelectric cantilevers may be millimeter-size cantilevers made by bonding a commercial PZT film to a non-piezoelectric substrate such as stainless steel, titanium or glass. Piezoelectric cantilevers have a number of advantageous properties, such as the capability of electrical self-excitation and self-sensing for in-situ electrical detection. Furthermore, piezoelectric cantilevers, when coated with an insulation layer, are capable of preventing conduction in liquid media, rendering the piezoelectric cantilevers suitable for detection of biomolecules in liquids.
US 2011/0086368 discloses a piezoelectric microcantilever sensor for assessing a patient's immunological response by measuring a resonance frequency shift of the sensor caused by binding of an immunological response factor to a corresponding receptor on the surface of the sensor. The microcantilever sensor may have a piezoelectric layer at its center, two electrodes, an encapsulating insulation layer and a receptor layer on the insulation layer. The microcantilever sensor may be treated with a mercaptopropyltrimethoxysilane (MPS) solution in ethanol at a pH of 4.5 to form the encapsulating insulation layer. The pH of the MPS solution may be adjusted to 4.5 using glacial acetic acid. In one embodiment, an epidermal growth factor receptor is attached to the MPS insulation layer through a bi-functional linker such as sulfo-SMCC (sulfosuccinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate). The microcantilever sensor is then exposed to a fluid for detection of epidermal growth factor in the fluid. An antibody may be bound to the microcantilever sensor for detecting an antigen.
WO 2009/126378 discloses a piezoelectric microcantilever sensor having enhanced detection sensitivity. The piezoelectric sensor includes a piezoelectric layer positioned between two conductive elements. For applications involving detection of a target in liquids, the sensor may be coated with an electrical insulating layer. The insulating layer may be either a coating of parylene(poly-para-xylylene) or self-assembled monolayers of methyltrimethoxysilane. U.S. Pat. No. 7,084,554 discloses a microcantilever, which comprise a piezoelectric thin PZT film of about 1-10 μm in thickness for the purpose of increasing the working frequency range of micro-electro-mechanical dimensioned systems. The patent further teaches that the piezoelectric thin film may be fabricated by thin film fabrication methods such as a sol-gel method, sputtering, hydrothermal methods, chemical vapor deposition or another thin film fabrication method, followed by low temperature annealing and dry etching, plasma etching or patterning by wet chemical etching.
Recent advances in thin-film piezoelectric PZT microcantilevers incorporate an electrical insulation layer that prevents liquid damping. US 2005/0112621 discloses a sensor system comprising a cantilever with a piezoelectric film and having one end fixed on a substrate, a piezoelectric capacitor for self-sensing and actuating on at least one side of an upper surface and a lower surface of the cantilever, a lower electrode formed at a lower surface of the piezoelectric film and an upper electrode formed at an upper surface of the piezoelectric film, an electric pad for applying electricity to the lower electrode and the upper electrode, and a molecular recognition layer formed at least one surface of the cantilever. The cantilever is taught to have an insulation layer surrounding the cantilever in order to prevent conduction in liquid media.
Piezoelectric plate sensors have been used to detect acceleration for the purpose of automotive posture control and seismic detection. US 2002/0078749 discloses an acceleration sensor comprising a first piezoelectric plate, a second piezoelectric plate bonded to the first piezoelectric plate by direct bonding, a first external electrode provided on the main surface of the first piezoelectric plate and a second external electrode provided on the main surface of the second piezoelectric plate. The first piezoelectric plate and second piezoelectric plates are bonded together with their polarization axes reversed relative to each other. The piezoelectric plate sensors however are not suitable for detecting biomolecules from a liquid sample.
Current piezoelectric-based sensors may lack the desired detection sensitivity necessary for many biomedical applications, particularly in-situ biosensing applications. These sensors typically have piezoelectric properties, characterized by a low piezoelectric coefficient d31 of less than 20 pm/v. The detection sensitivity of piezoelectric cantilever sensors, which may be viewed as simple harmonic oscillators, is correlated to the resonance frequency shift capability of the sensor. The resonance frequency shift capability in turn is dependent upon the sensor's ability to respond to changes in the effective spring constant and effective mass of the sensor. Available piezoelectric-based sensors, such as piezoelectric microcantilevers constructed from bulk PZT of relatively large thickness, are only useful for detecting relatively large changes in the effective spring constant of the sensor.
Improving sensitivity, accuracy and efficiency of piezoelectric sensors for detecting biomolecules is important to the development of sensitive and reliable assays in the healthcare field for early detection and prevention of diseases. For example, early diagnosis of breast cancer, especially when the tumor is still small is important to the prognosis of the patient. However, diagnosis of early breast cancer by mammography is frequently inadequate and often produces false positives leading to unnecessary biopsies and stress to misdiagnosed patients. Diagnosing breast cancer using breast cancer biomarkers, such as HER2 in serum has gained significant attention. Therefore early detection methods for various cancers and other diseases that allow accurate, effective and non-invasive identification and quantification of disease biomarkers and pathogens are needed.
To address these issues, in one embodiment, the present invention provides a piezoelectric plate sensor (PEPS) capable of in-solution detection of biomolecules with a zeptomolar or higher sensitivity.
In addition, there remains a need for an array of sensors with high sensitivity and that is capable of simultaneous detection of multiple biomolecules in a sample.